Burnout, Black Lines Matter?

Back end is trying to pass the front, any way it can.

Gravity and Road Crown determins the direction left or right.

Being you are saying it is swinging left, (which I would think it would be swinging right if it were from road crown).

Here is a thought:
I know you have a Sure-grip but as compared to an open diff (one wheel wonder) when you give it the go the right rear wheel lifts from the rotational torque of the drive line, and it will smoke that right tire because it has less weight on it and the left tire is still planted firm.

So follow along: with the right tire slightly lifting with your Sure-grip the tire diameter increases by maybe 1/2". Then with the weight transfer to the back of the car from the acceleration it is planting the load on to the left tire, actually decreasing it's diameter.

Add up the 2 differences of the 2 tires under power, and it is naturally going to want to go left.

They sell the Super Stock springs that are rated left and right. (they give a very stiff ride so be warned) The left spring has a little more arch to it (or is slightly stiffer) to help preload the right rear tire to help offset the driveline rotational force.

Also there are what is called the "Big Block" short lower leaf spring that helps the rear end from wrapping up. They are different lengths from left to right to also help offset the twisting/lifting forces under acceleration.